Fiber-optic amplifiers of the cited type are known, e.g. from: B. Clesca, et al, "32 channel, 48 dB CNR and 46 dB budget AM-VSB transmission experiment with field-ready post-amplifiers" in Optical Amplifiers and Their Applications, 1992 Technical Digest (Optical Society of America, Washington, D.C., 1992), Vol. 17, pages 91 to 94. They show a fiber-optic amplifier which has two pump light sources that emit pump light at a wavelength of 1480 nm, two pump light couplers and one section of an optical waveguide that is doped with erbium ions.
In optical transmission systems with optical waveguides, fiber-optic amplifiers serve to amplify the optical signals in optical waveguides.
Such transmission systems have the requirement e.g. that the transmission line is safe for the eyes, i.e. that a person's eyes are not endangered when looking intentionally or unintentionally into an interrupted optical waveguide. To ensure this safety requirement, the system, or individual components of the system, must be turned off in the event of a fiber break.
An interruption of the transmission line can be determined e.g. by monitoring the input light of a fiber-optic amplifier. Missing input light could mean that the transmission line has been interrupted before this fiber-optic amplifier.
Monitoring the input light of a fiber-optic amplifier is known from the above cited literature. Therein a part of the input light is coupled out through a coupler, which is part of the fiber-optic amplifier, and detected by a photodiode. The signal-to-noise ratio deteriorates because the amplifying part of the fiber-optic amplifier has less light power available, and the insertion of the coupler causes additional damping. Furthermore, this coupler causes additional expense for material and labor time.